Shock loads change the resistance, resiliency, and productivity of anaerobic co-digestion of municipal sludge and fats, oils, and greases

Abstract

Accidental organic overloading (shock loading) is a common cause of failure in the anaerobic co-digestion of fats, oils, and greases (FOG). Nonetheless, questions still remain about how shock loads alter an anaerobic digester's biogas productivity and response to future organic overloads. To address this knowledge gap, this study utilized short increases in OLR, or shock loads, to adapt one reactor to FOG co-digestion and compare its response to that of a non-adapted digester tested at similar organic loads. A long-term study was also conducted where sequential shock events and disturbance-to-failure events (with organic loading rates ranging from 2.5 to 9.0 g VS/L/d) were employed to study their effects on reactor stability and performance. When exposed to moderate and large shock loads (3.0 and 9.0 g VS/L/d, respectively), the anaerobic digester previously exposed to FOG shock loads had greater levels of resistance and resiliency. For a moderate shock, it took the non-adapted reactor 29.8, 43, 44, and 19 days longer than the adapted reactor to recover to the baseline levels for methane content, organic acids (OA), pH, and mcrA concentrations, respectively. For a large shock, the adapted and non-adapted reactors saw more similar recovery times with the non-adapted reactor requiring an additional 11, 18, 19, and 0 days longer than the adapted reactor to recover to baseline levels for methane content, OA, pH, and mcrA concentrations, respectively. However, exposure to successive large shock loads decreased the anaerobic digester's resistance and resilience. The length of time it took for the organic acid accumulation and methane content values to return to their pre-shock values generally increased with each successive large shock load. For OA, the reactor required 35, 39, and 43 days to recover from the first, second, and third large shocks, respectively, while the methane content recovery times were 27.5, 26.5, and 34.5 days for the same shock periods. Thus, this work demonstrates that there is a tipping point in which FOG shock loads, whether intentional or accidental, go from improving the overall robustness of an anaerobic digester to significantly deteriorating its overall performance.